In performing various machining operations using a numerical controlled machine tool, the movement of the tool with respect to a workpiece is shifted from a rapid traverse to a machining feed or vice versa by the movement of X, Y and Z axes of the machine. Such machining operation is exemplified by a boring operation by the movement of the Z-axis as a simple example.
FIGS. 15a and 15b illustrate the boring operation performed by the Z-axis motion in a conventional manner. FIG. 15a shows a moving path of a drill 31a. The axial motion by the rapid traverse is indicated by a dotted arrow, and the axial motion by a cutting feed is indicated by a solid arrow. Any motion is caused by the movement of the Z axis. A spindle is rotating at a previously commanded rotational speed.
In a section 81, a tip of the drill 31a moves closer to a surface of the workpiece. In a section 82, the drill tip moves to the machining position of a bottom of the bore by cutting feed at the commanded speed. In a section 83, the drill tip moves to a start point of the movement of the section 82 by the rapid traverse. In a section 84, the drill tip moves to a start point of the movement of the section 81 by the rapid traverse.
FIG. 15b is a diagram showing the movement in the Z-axis direction in the form of a speed wave. The vertical axis represents the speed, and the horizontal axis represents the time; the "+" direction of the vertical axis indicates the direction in which a spindle head is raised.
In the section 81, the spindle head moves downwardly in the Z-axis direction as accelerated up to the rapid traverse speed and then keeps traveling at the rapid traverse speed. Then, the movement of the spindle head is decelerated until the speed becomes "0" again. The accelerating time from "0" to the rapid traverse speed and the decelerating time from the rapid traverse speed to "0" are set by parameters for defining time constants for acceleration/deceleration. The acceleration/deceleration time constant is controlled to an appropriate value so as not to give an impact to the machine. In this example, the maximum moving speed and the acceleration/deceleration time constant are set to such values that can be set for a numerical controlled machine tool capable of a relatively high-speed movement. In the section 82, the spindle head moves downwardly in the Z-axis direction at the commanded feeding speed while executing the cutting operation.
In the section 83, the spindle head moves to the position of the Z coordinate at which the machining has started by the rapid traverse motion. At that time, since the extent of movement is small, the Z-axis moving speed is shifted to deceleration during acceleration without reaching the rapid traverse speed. In the section 84, the spindle head moves to the start point of the section 81 by rapid traverse motion.
Upon termination of deceleration of each movement, it is confirmed that the spindle head has entered the tolerance of the commanded position, and then the spindle head starts the next movement.
A conventional example in which a workpiece is cut into a predetermined profile by moving the workpiece relative to the tool as the X-, Y- and Z-axis motors will be described. FIG. 16 shows a cutting of a workpiece into a predetermined profile according to the conventional axial movement. In this conventional example, the workpiece 21a is cut horizontally by a drill or cutter 31a. During that time, the cutter 31a moves in a first section 1a from a point P11 to a point P12 by a straight rapid traverse; a second section 2a from the point P12 to a point P13 by the straight rapid traverse; in a third section 3a from the point P13 to a point 14 by the straight cutting feed; a fourth section 4a from the point P14 to a point P15 by the straight cutting feed, and then in a fifth section 5a from the point P15 to a point 16 again by the straight rapid traverse. The rapid traverse in this example is carried out by a positioning motion by a straight-line interpolation, that is, the positioning motion with a substantially straight moving path. Accordingly, the successive points are connected by straight lines.
In the conventional axial moving method, the speed always becomes "0" at both of the start and end points of the movement. Accordingly, a wasteful interruption is repeated very frequently in the machining of the workpiece, which would be a cause for extending the machining time, thus lowering the productivity of the machine.
Frequent acceleration/deceleration more than necessary increases the quantity of heat generated by the motor, to cause overheating. If the motor overheats, machining has to be temporarily stopped, which would lower the productivity of the machine. Further, if the acceleration/deceleration is set with an increased tolerance of heat so as to avoid overheating, adequate performance of the motor can not be achieved, thus lowering the rate of production.